Technological development and increased demand for mobile equipment have led to a rapid increase in demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle span and low self-discharge are commercially available and widely used.
Lithium-containing cobalt oxide (LiCoO2) is generally used as a cathode active material for lithium secondary batteries. Use of lithium-containing manganese oxides such as LiMnO2 having a layered-crystal structure and LiMn2O4 having a spinel-crystal structure, and lithium-containing nickel oxide (LiNiO2) is also considered.
Among these cathode active materials, LiCoO2 is the most generally used owing to superior physical properties such as superior cycle characteristics, but has low stability and is costly due to resource limitations of cobalt as a raw material.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 advantageously use, as a raw material, manganese which is abundant and eco-friendly, thus attracting much attention as a cathode active material alternative to LiCoO2. However, these lithium manganese oxides have disadvantages of low capacity and bad cycle characteristics.
In addition, lithium nickel oxides such as LiNiO2 are cheaper than cobalt oxides and have higher discharge capacity, when charged to 4.25V. More specifically, doped LiNiO2 has a reversible capacity of about 200 mAh/g which is higher than LiCoO2 capacity (about 153 mAh/g). Accordingly, in spite of slightly low average discharge voltage and volumetric density, commercial batteries comprising LiNiO2 as a cathode active material have improved energy density and a great deal of research into these nickel-based cathode active materials is thus actively conducted in order to develop high-capacity batteries recently.
In this regard, many conventional techniques focus on properties of LiNiO2-based cathode active materials and improvement in preparation process of LiNiO2 and suggest lithium transition metal oxides wherein nickel is partially substituted by other transition metal such as Co or Mn. However, the problems of LiNiO2-based cathode active materials including high preparation costs, swelling caused by gas generation in batteries, low chemical stability and high pH have been satisfactorily not yet solved.
Accordingly, in the related art, there is an attempt to improve performance of batteries by applying a material such as LiF, Li2SO4 or Li3PO4 to the surface of lithium nickel-manganese-cobalt oxide. In this case, the substance is disposed only on the surface of the lithium nickel-manganese-cobalt oxide, thus disadvantageously having a limitation on exertion of effects to a desired level and requiring a separate process for application of the material to the surface thereof.
However, in spite of such various attempts, lithium composite transition metal oxide exhibiting satisfactory performance has yet to be developed.